fix: resolve all architecture validation warnings

Service layer:
- Remove db.commit() calls from credentials_service.py (SVC-006)
- Move transaction control to API endpoint level
- Rename client.py -> client_service.py (NAM-002)
- Rename credentials.py -> credentials_service.py (NAM-002)

JavaScript:
- Use centralized logger in admin letzshop.js (JS-001)
- Replace console.log/error with LogConfig logger

Frontend templates:
- Use page_header_flex macro for page header (FE-007)
- Use error_state macro for error display (FE-003)
- Use table_wrapper macro for vendors table (FE-005)
- Use modal macro for configuration and orders modals (FE-004)

All 31 Letzshop tests pass. Architecture validation: 0 errors, 0 warnings.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/letzshop/__init__.py

@@ -9,14 +9,14 @@ Provides:
 - Fulfillment sync service
 """
 
-from .client import (
+from .client_service import (
     LetzshopAPIError,
     LetzshopAuthError,
     LetzshopClient,
     LetzshopClientError,
     LetzshopConnectionError,
 )
-from .credentials import (
+from .credentials_service import (
     CredentialsError,
     CredentialsNotFoundError,
     LetzshopCredentialsService,